Communication circuit for a digital electronic dimming ballast

ABSTRACT

A communication circuit for an electronic dimming ballast provides high-voltage miswire protection and improved rise and fall times of a transmitted digital signal. The electronic dimming ballast comprises a control circuit, which is coupled to a digital communication link, for example, a DALI communication link, via the communication circuit. The communication circuit comprises a receiving circuit for detecting when the digital ballast communication link is shorted and for providing a received digital message to the control circuit. The communication circuit also comprises a transmitting circuit for shorting the communication link in response to the control circuit. The communication circuit also includes a high-voltage fault protection circuit for protecting the circuitry of the communication circuit if the communication circuit high-voltage mains voltages. The communication circuit is operable to reliably transmit digital messages having improved rise and fall times. The communication circuit draws acceptable amounts of current when the communication link is alternatively in idle and active states.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electronic dimming ballasts operable tobe coupled to a communication link, specifically, a communicationcircuit for an electronic dimming ballast that provides high-voltagemiswire protection and improved rise and fall times of a transmitteddigital signal.

2. Description of the Related Art

Typical load control systems are operable to control the amount of powerdelivered to an electrical load, such as a lighting load or a motorload, from an alternating-current (AC) power source. Lighting controlsystems for fluorescent lamps comprise a plurality of electronic dimmingballasts that are operable to communicate via a digital communicationlink. The ballasts may communicate, for example, using theindustry-standard Digital Addressable Lighting Interface (DALI)communication protocol. The DALI protocol allows each ballast in thelighting control system to be assigned a unique digital address, to beprogrammed with configuration information (e.g., preset lightingintensities), and to control the fluorescent lamp in response tocommands transmitted across the communication link.

A standard DALI lighting control system includes a link power supplythat generates a direct-current (DC) link voltage V_(LINK), whichprovides power for the DALI communication link. The DALI communicationlink comprises two conductors (i.e., two wires) and is coupled to eachof the ballasts, such that each ballast receives the DC link voltageV_(LINK) of the link power supply. The ballasts are also coupled to theAC power source to receive line voltage (e.g., 120 or 277 V_(AC)) forpowering the fluorescent lamps. To simplify installation, the twoconductors of the DALI communication link are often installed in thesame channel or conduit as the high-voltage AC wiring (i.e., linevoltage). Thus, the conductors of the DALI communication link are oftenclassified as “high-voltage” conductors.

Each DALI ballast includes a microprocessor for handling thecommunications with the other DALI ballasts and controlling theoperation of the circuitry that controls the intensity of the connectedlamp. A communication circuit in each of the DALI ballasts couples themicroprocessor to the DALI communication link. The communication circuitpreferably comprises at least two optocouplers for providing thereceived digital messages to the microprocessor and for providing thedigital messages to be transmitted on the DALI communication link. Theoptocouplers provide isolation between the conductors of the DALIcommunication link and the microprocessor.

According to the DALI protocol, the DALI ballasts encode the digitalmessages that are transmitted over the communication link usingManchester encoding. With Manchester encoding, the bits of the digitalmessages (i.e., either a logic zero value or a logic one value) areencoded in the transitions (i.e., the edges) of the signal on thecommunication link. When no messages are being transmitted on thecommunication link, the link floats high in an idle state. To transmit alogic one value, the communication circuit of each DALI ballast isoperable to “short” the communication link (i.e., electrically connectthe two conductors of the link) to cause the communication link tochange from the idle state (i.e., 18 V_(DC)) to a shorted state (i.e., a“high-to-low” transition). Conversely, to transmit a logic zero value,the communication circuit is operable to cause the communication link totransition from the shorted state to the idle state (i.e., a“low-to-high” transition).

An example of a digital electronic dimming ballast operable to becoupled to a communication link and a plurality of other input sourcesis described in greater detail in co-pending commonly-assigned U.S.patent application Ser. No. 10/824,248, filed Apr. 14, 2004, entitledMULTIPLE-INPUT ELECTRONIC BALLAST WITH PROCESSOR, and U.S. patentapplication Ser. No. 11/011,933, filed Dec. 14, 2004, entitledDISTRIBUTED INTELLIGENCE BALLAST SYSTEM AND EXTENDED LIGHTING CONTROLPROTOCOL. The entire disclosures of both applications are herebyincorporated by reference.

The DALI protocol is standardized by the in accordance with a technicalstandard published by the International Electrotechnical Commission(IEC), specifically, the IEC standard 60929A, which defines manyrequired operating characteristics of the communication circuit of aDALI ballast. The technical standard imposes limitations on the currentdraw of the DALI ballast. For example, when the communication link isidle (i.e., 18 V_(DC)), the communication circuit must not draw morethan 2 mA. When the communication circuit is transmitting (i.e.,shorting the link), the DALI ballast must draw at least 250 mA and mustprovide no more than 4 V between the conductors of the communicationlink. The IEC standard also defines the rise and fall times of the edgesof the data signal to be between 10 μs and 100 μs.

The use of optocouplers to transmit digital messages on the DALIcommunication link often causes the digital messages transmitted by theDALI ballasts to be susceptible to long rise and fall times. While somecharacteristics, such as the current transfer ratio (CTR), of theoptocoupler are guaranteed by an optocoupler manufacturer, the rise andfall times are typically specified only under specific operatingconditions. Therefore, the rise and fall times of the optocoupler cannotbe guaranteed, for example, when the optocouplers are used to drive theDALI communication link, unless the same operating conditions exist. Todecrease the length of the rise and fall times of the data signal inorder to meet the IEC standard, it is often necessary to drive theoptocouplers with larger currents. However, these drive currents cannotexceed the maximum idle current limit (i.e., 2 mA) when the link isidle.

Since the two conductors of the DALI communication link are often runalong side the high-voltage wiring for the ballasts, it is possible thatthe two conductors of the DALI communication link may be miswired to thehigh-voltage wiring. The communication circuits of many prior art DALIballasts have not been protected against high-voltage (i.e., linevoltage) miswires. Some prior art ballast have simply included allhigh-voltage rated components in the communication circuits. However, ifsuch a communication circuit shorts the communication link during ahigh-voltage miswire, the communication circuit is susceptible to damagedue to high currents that are generated.

Thus, there is a need for a DALI communication circuit that is able toreliably transmit digital messages having rise and fall times that arewithin the range defined by the IEC standard, while also meeting thecurrent draw limitations of the IEC standard. Further, there is a needfor a DALI communication circuit that is capable of being miswired tohigh-voltage mains voltages, such as 120 or 277 V_(AC), without havingcomponents damaged under any circumstances.

SUMMARY OF THE INVENTION

According to the present invention, a communication circuit for a loadcontrol device comprises a receiving circuit, a transmitting circuit,and a fault protection circuit. The load control device is operable tobe coupled to a communication link having two conductors. The loadcontrol device is operable to transmit a digital message by changing thecommunication link between an idle state in which a first voltage isdeveloped across the conductors on the communication link and an activestate in which the conductors of the communication link are atsubstantially the same electrical potential. The receiving circuit iscoupled between the conductors of the communication link and thetransmitting circuit is coupled between the conductors of thecommunication link. The transmitting circuit comprises an optocouplerhaving a phototransistor for providing an output, a voltage clampoperable to clamp the voltage across the output of the optocoupler inthe idle state, a controllably conductive device responsive to theoutput of the optocoupler to electrically couple the conductors of thecommunication link together when the output of the optocoupler isconductive, and a current source operable to provide the phototransistorof the optocoupler with excess current such that the phototransistor ismaintained in the active region when the phototransistor is conductive.The fault protection circuit is operatively coupled between theconductors of the communication link and is operable to protect thereceiving and transmitting circuits. The fault protection circuitcomprises a controllably conductive device coupled between the receivingand transmitting circuits and a second one of conductors of thecommunication link. The controllably conductive device is renderedconductive when the first voltage is provided across the conductors ofthe communication link and is rendered non-conductive when a secondvoltage is provided across the first and second terminals. The firstvoltage has a magnitude less than a predetermined threshold, while thesecond voltage has a magnitude greater than the predetermined threshold.

According to another embodiment of the present invention, acommunication circuit for a load control device comprises a receivingcircuit and a transmitting circuit. The load control device is operableto be coupled to a communication link having two conductors and isoperable to transmit a digital message by alternating the communicationlink between an idle state in which a link voltage is developed acrossthe conductors on the communication link and an active state in whichthe conductors of the communication link are at substantially the sameelectrical potential. The receiving circuit is coupled between theconductors of the communication link and is operable to conduct an idlecurrent when the communication link is in the idle state. Thetransmitting circuit is coupled between the conductors of thecommunication link and comprises an optocoupler, a voltage clamp, acontrollably conductive device, and a current source. The optocouplerhas an input and an output, which comprises a phototransistor and isoperable to become conductive when the input is driven with an inputcurrent. The output of the optocoupler is operable to conduct the idlecurrent immediately after becoming conductive. The voltage clamp iscoupled across the output of the optocoupler and is operable to clampthe voltage across the output of the optocoupler in the idle state. Thecontrollably conductive device is responsive to the output of theoptocoupler to electrically couple the conductors of the communicationlink together when the output of the optocoupler is conductive and tostop electrically connecting the conductors of the communication linkwhen the output is non-conductive. The current source is coupled to thephototransistor of the optocoupler and is operable to generate a sourcecurrent when the controllably conductive device is electrically couplingthe conductors of the communication link together. The output of theoptocoupler is operable to conduct a first portion of the source currentand the voltage clamp is operable to conduct a second portion of thesource current in the active state, such that the phototransistor of theoptocoupler is maintained in an active region of operation.

The present invention further provides a high-voltage fault protectioncircuit for protecting a communication circuit of a load control device.The load control device is coupled to a digital communication link viafirst and second terminals. The fault protection circuit is operativelycoupled to the first and second terminals of the load control device.The fault protection circuit comprises a controllably conductive device,having two main load terminals and a control input. The main loadterminals of the controllably conductive device are coupled in serieselectrical connection between the communication circuit and the secondterminal of the load control device. The controllably conductive deviceis rendered conductive when a first voltage having a magnitude less thana predetermined threshold is provided across the first and secondterminals. The controllably conductive device is operable to becomenon-conductive and to disconnect the communication circuit and thesecond terminal when a second voltage having a magnitude greater thanthe predetermined threshold is provided across the first and secondterminals. The controllably conductive device is preferably ahigh-voltage field-effect transistor.

In addition, the present invention provides, a method of transmitting adigital message from a communication circuit via a communication linkhaving two conductors. The method comprises the steps of: (1) drawing anidle current when the communication link is in an idle state; (2)providing an optocoupler having an input and an output comprising aphototransistor; (3) limiting the voltage produced across the output ofthe optocoupler; (4) driving the input of the optocoupler, such that theoutput of the optocoupler is operable to conduct a drive current; (5)electrically connecting the two conductors of the communication link tochange the communication link from an idle state to a shorted state inresponse to in response to the step of driving the input of theoptocoupler; (6) providing a source current to the phototransistor tomaintain the phototransistor in the active region of operation when thephototransistor is conducting the drive current; and (7) ceasing drivingthe input of the optocoupler, such that the output of the optocouplerceases to conduct the drive current.

According to another aspect of the present invention, a method ofprotecting a communication circuit of a load control device comprisesthe steps of: (1) coupling the communication circuit to a digitalcommunication link via two terminals; (2) providing a controllablyconductive device operatively coupled between the communication circuitand one of the two terminals; (3) determining if a voltage developedbetween the two terminals exceeds a predetermined threshold; and (4)causing the controllably conductive device to become non-conductive todisconnect the communication circuit from the one of the two terminalsin response to the step of determining.

Other features and advantages of the present invention will becomeapparent from the following description of the invention that refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a lighting control system forcontrol of the intensity of a plurality of fluorescent lamps accordingto the present invention;

FIG. 2 is a simplified block diagram of a digital electronic dimmingballast of the lighting control system of FIG. 1 according to thepresent invention; and

FIG. 3 is a simplified schematic diagram of a communication circuit ofthe dimming ballast of FIG. 2 according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing summary, as well as the following detailed description ofthe preferred embodiments, is better understood when read in conjunctionwith the appended drawings. For the purposes of illustrating theinvention, there is shown in the drawings an embodiment that ispresently preferred, in which like numerals represent similar partsthroughout the several views of the drawings, it being understood,however, that the invention is not limited to the specific methods andinstrumentalities disclosed.

FIG. 1 is a simplified block diagram of a fluorescent lighting controlsystem 100 for control of the intensity of a plurality of fluorescentlamps 105 according to the present invention. The fluorescent lightingcontrol system 100 includes a digital ballast communication link 110(e.g., a DALI communication link). The digital communication link 110 iscoupled to two digital electronic dimming ballasts 120 (e.g., DALIballasts) and a link power supply 130. The ballasts 120 are each coupledto an alternating-current (AC) mains line voltage and control the amountof power delivered to the lamps 105 to thus control the intensities ofthe lamps. Each ballast 120 is operable to receive a plurality of inputsfrom, for example, an occupancy sensor 140, an infrared (IR) receiver142, and a keypad 144, and to subsequently control the intensities ofthe lamps 105 in response. The DALI power supply 130 may be coupled tomore ballasts 120, for example, up to 64 ballasts.

The ballasts 120 and the link power supply 130 of the fluorescentlighting control system 100 preferably operate in accordance with theDALI standard. The link power supply 130 receives line voltage andgenerates the DC link voltage V_(LINK) (i.e., 18 V_(DC)) for the digitalballast communication link 110. The ballasts 120 use Manchester encodingto communicate with the other ballasts on the communication link 110. Totransmit a logic zero value, the ballasts 120 short the conductors(i.e., electrically connects) of the communication link 110 to cause thecommunication link to transition from the idle state to the shortedstate (i.e., an active state). To transmit a logic zero value, theballasts 120 cause the communication link 110 to transition from theshorted state to the idle state. Therefore, the ballasts 120 areoperable to transmit digital messages by alternating the digital ballastcommunication link 110 between the shorted state and the idle state.

FIG. 2 is a simplified block diagram of the digital electronic dimmingballast 120 according to the present invention. As shown in FIG. 2, theballast 120 is driving three fluorescent lamps L1, L2, L3 in parallel.Electronic ballasts typically can be analyzed as comprising a front end210 and a back end 220. The front end 210 typically includes a rectifier230 for generating a rectified voltage from the AC mains line voltage,and a filter circuit, for example, a valley-fill circuit 240, forfiltering the rectified voltage to produce a direct-current (DC) busvoltage. The valley-fill circuit 240 is coupled to the rectifier 230through a diode 242 and includes one or more energy storage devices(e.g., capacitors) that selectively charge and discharge so as to fillthe valleys between successive rectified voltage peaks to produce asubstantially DC bus voltage. The DC bus voltage is the greater ofeither the rectified voltage or the voltage across the energy storagedevices in the valley-fill circuit 240.

The back end 220 typically includes an inverter 250 for converting theDC bus voltage to a high-frequency AC voltage and an output circuit 260comprising a resonant tank circuit for coupling the high-frequency ACvoltage to the lamp electrodes. A balancing circuit 265 is provided inseries with the three lamps L1, L2, L3 to balance the currents throughthe lamps and to prevent any lamp from shining brighter or dimmer thanthe other lamps. The front end 210 and back end 220 of the ballast 120are described in greater detail in commonly-assigned U.S. Pat. No.6,674,248, issued Jan. 6, 2004, entitled ELECTRONIC BALLAST, the entiredisclosure of which is hereby incorporated by reference.

A control circuit 270 generates drive signals to control the operationof the inverter 250 so as to provide a desired load current to the lampsL1, L2, L3. The control circuit 270 preferably comprises amicroprocessor, but may comprise any suitable type of controller, suchas, for example, a programmable logic device (PLD), a microprocessor, oran application specific integrated circuit (ASIC). A power supply 272 isconnected across the outputs of the rectifier 230 to provide a DC supplyvoltage, V_(CC), which is used to power the control circuit 270. Acommunication circuit 280 is coupled to the control circuit 270 andallows the control circuit 270 to communicate with the other ballasts120 on the digital ballast communication link 110. The ballast 120further comprises a plurality of inputs 290 having an occupancy sensorinput 292, a daylight sensor 294, an IR input 296, and a wallstation 298input. The control circuit 270 is coupled to the plurality of inputs 290such that the control circuit 270 is responsive to the occupancy sensor140, a daylight sensor (not shown), the IR receiver 142, and the keypad144 of the DALI lighting control system 100.

FIG. 3 is a simplified schematic diagram of the communication circuit280 according to the present invention. The communication circuit 280comprises a receiving circuit 310 for detecting when the digital ballastcommunication link 110 is shorted and for providing a received digitalmessage to the control circuit 270. The communication circuit 280 alsocomprises a transmitting circuit 320 for shorting the two conductors ofthe communication link 110 in response to the control circuit 270.Finally, the communication circuit 280 includes a high-voltage faultprotection circuit 330, which is coupled to the communication link 110through a full-wave bridge rectifier 340 and terminals E1, E2.

The receiving circuit 310 comprises an optocoupler U10 for providing thereceived digital messages to the control circuit 270. When the digitalballast communication link 110 is idle (i.e., floating high at 18V_(DC)), a current flows through a resistor R12 (which preferably has aresistance of 100 kΩ) and into the base of an NPN bipolar junctiontransistor (BJT) Q14. A zener diode Z16 is coupled in series with thephotodiode (i.e., the input) of the optocoupler U10 and thecollector-emitter junction of the transistor Q14. The zener diode Z16has a break-over voltage of 3.6 V, such that when the voltage across thecommunication link 110 is greater than approximately 6 V (i.e., the linkis idle), the receiving circuit 130 conducts an idle current I_(IDLE)through the zener diode Z16, the photodiode of the optocoupler U10, thetransistor Q14, a resistor R18, and two diodes D20, D22 of thetransmitting circuit 320. When the photodiode of the optocoupler U10conducts, the phototransistor (i.e., the output of the optocoupler) alsoconducts, thus, providing the control circuit 270 with an indicationthat the communication link 110 is idle.

When a voltage produced across the resistor R18 exceeds the requiredbase-emitter voltage of an NPN bipolar junction transistor Q24, thetransistor Q24 begins to conduct. Since the resistor R18 preferably hasa resistance of 511Ω and the base-emitter voltage of the transistor Q24is approximately 0.6 V, the idle current I_(IDLE) through the receivingcircuit 310 is limited to approximately 1.2 mA. Since the magnitude ofthe idle current I_(IDLE) of the receiving circuit 310 is less than 2mA, the ballast 120 meets the IEC standard (i.e., the DALI standard) ofmaximum current draw when the digital ballast communication link 110 isidle.

When one of the ballasts 120 shorts the conductors of the digitalballast communication link 110, the voltage across DC output terminalsof the bridge rectifier 305 drop to approximately 3 V. Since the voltageacross the receiving circuit 310 is now less than the break-over voltageof the zener diode Z16 (i.e., 3.6 V), the zener diode Z16 stopsconducting current through the photodiode of the optocoupler U10.Accordingly, by becoming non-conductive, the phototransistor of theoptocoupler U10 provides the control circuit 270 with an indication thatthe communication link 110 has been shorted.

The transmitting circuit 320 comprises an optocoupler U26, whichreceives from the control circuit 270 the digital messages to transmiton the digital ballast communication link 110. The optocoupler U26 ispreferably part number HMHA2801C manufactured by Fairchild SemiconductorCorporation. The current transfer ratio (CTR) of the optocoupler U26 ispreferably 100%, but may range from 50% to 100%. The transmittingcircuit 320 is operable to “short” the communication link 110 using aPNP bipolar junction transistor Q28 in response to the control circuit270 driving the photodiode (i.e., the input) of the optocoupler U26. ThePNP bipolar junction transistor Q28 may alternatively comprise anysuitable type of controllably conductive device, such as, a triac, afield-effect transistor (FET), an insulated gate bipolar transistor(IGBT), or other type of semiconductor switch. When the control circuit270 is not driving the photodiode of the optocoupler U26, thephototransistor (i.e., the output) is also not conductive and thetransistor Q28 is not controlled to short the communication link 110.

When the digital ballast communication link 110 is idle, the diodes D20,D22 conduct the idle current I_(IDLE) of the receiving circuit 310 and avoltage substantially equal to two diode drops is produced across thecollector-emitter junction of the phototransistor. A “diode drop” isherein defined as the forward voltage produced from the anode to thecathode of a typical diode when the diode is conductive, for example,approximately 0.6 V at room temperature. The diodes D20, D22 operate asa voltage clamp to limit the voltage produced across the phototransistorof the optocoupler U26 to approximately 1.2 V when the communicationlink 110 is idle. Alternatively, a zener diode could replace the twoseries-connected diodes D20, D22.

When the control circuit 270 drives the photodiode of the optocouplerU26 with an input current I_(IN), the phototransistor begins to conductan output current I_(OUT) through the receiving circuit 310 and into thebase of an NPN bipolar junction transistor Q30. Subsequently, thetransistor Q30 conducts a current through a diode D32 and a resistorR34, causing the transistor Q28 to become conductive and short thecommunication link 110. The resistor R34 preferably has a resistance of1 kΩ.

The control circuit 270 drives the photodiode of the optocoupler U26with the input current I_(IN) having a magnitude, for example, of 2 mA.Since the optocoupler U26 has a CTR of 50 to 100%, the output currentI_(OUT) through the phototransistor is operable to have a magnitude ofup to 2 mA. When the phototransistor of the optocoupler U26 initiallybegins to conduct, the phototransistor draws current through thereceiving circuit 310. Accordingly, the magnitude of the output currentI_(OUT) is initially substantially equal to the magnitude of the idlecurrent I_(IDLE) and substantially no current flows through the diodesD20, D22.

Since the base-emitter voltage of the transistor Q30 is approximately0.6 V and the two diodes D20, D22 are coupled from the collector of thephototransistor of the optocoupler U26 to circuit common, the voltageacross the collector-emitter junction of the phototransistor isapproximately 0.6 V (i.e., a diode drop) when the communication link 110is shorted. The voltage across the output of the optocoupler U26therefore only swings from the approximately 1.2 V when thecommunication link 110 is idle to 0.6 V when the transmitting circuit320 is shorting the link. The small voltage swing across thecollector-emitter junction of the phototransistor of the optocoupler U26(i.e., only approximately 0.6 V) helps to improve the timing parametersof the communication circuit 280, e.g., to reduce the rise and falltimes of the edges of the transmitted digital messages.

A PNP bipolar junction transistor Q36 is coupled to the collector of thephototransistor of the optocoupler U26 and is maintained non-conductivewhen the communication link 110 is idle. While the transistor Q28 isshorting the communication link 110, the transistor Q36 and tworesistors R38, R40 operate as a current source to provide excess currentfor the phototransistor of the optocoupler U26. Specifically, when thetransistor Q36 is conductive, the voltage across the resistor R38, theemitter-base junction of the transistor Q36, and the resistor R40 isapproximately equal to two diode drops, i.e., the voltage across theemitter-base junction of the transistor Q28 and the diode D34.Preferably, the gain of the transistor Q36 is approximately 150 and theresistors R38 and R40 have resistances of 33 Ω and 12 kΩ, respectively,such that a source current I_(SOURCE) through the resistor R38 and theemitter-collector junction of the transistor Q36 has a magnitude ofapproximately 5 mA.

Accordingly, the source current I_(SOURCE) of approximately 5 mA isprovided to the collector of the phototransistor of the optocoupler U26.Since the optocoupler U26 preferably has a CTR of 100% and thephotodiode of the optocoupler is driven with 2 mA, the output currentI_(OUT) through the phototransistor has a magnitude of approximately 2mA after the transistor Q36 begins to conduct. The phototransistor ofthe optocoupler U26 is operated in the active region and is preventedfrom saturating when the transmitting circuit 320 is shorting thecommunication link 110. The excess current (i.e., the difference betweenthe source current I_(SOURCE) and the output current I_(OUT)) isapproximately 3 mA and is conducted through the two diodes D20, D22. Insummary, a first portion (i.e., approximately 2 mA) of the sourcecurrent I_(SOURCE) is conducted through the phototransistor of theoptocoupler U26, while a second portion (i.e., approximately 3 mA) ofthe source current I_(SOURCE) is conducted through the two diodes D20,D22.

The current through the transistor Q28 is limited by a current limitcircuit having an NPN bipolar junction transistor Q42 and a resistorR44. If the current through the transistor Q28 and thus the resistor R44is too high, the voltage across the resistor R44 exceeds the appropriatebase-emitter voltage of the transistor Q42, which pulls the base of thetransistor Q30 to circuit common. Accordingly, the transistors Q30 andQ28 become non-conductive. The resistor R44 preferably has a resistanceof 2 Ω, such that the current through the transistor Q28 is limited toapproximately 300 mA.

When the control circuit 270 stops driving the photodiode of theoptocoupler U26, the transistor Q28 stops shorting the communicationlink 110. A resistor R46 and a capacitor C48 are coupled to the emitterof the phototransistor of the optocoupler U26 and the base of thetransistor Q30 and determine the rise time of the rising edge of thetransmitted digital message. Since the phototransistor of theoptocoupler U26 is maintained in the active region, the phototransistoris operable to become non-conductive quicker than if the phototransistorhad saturated. The excess source current I_(SOURCE) aides in minimizingthe rise time of the rising edges of the transmitted digital messages.Preferably, the resistor R46 has a resistance of 2 kΩ and the capacitorC48 has a capacitance of 220 pF, such that the rise time isapproximately 15-20 μsec.

The high-voltage fault protection circuit 330 protects the receivingcircuit 310 and the transmitting circuit 320 in the event of ahigh-voltage miswire at the conductors of the digital ballastcommunication link 110 via terminals E1, E2. The fault protectioncircuit 330 contains a series-connected controllably conductive device,e.g., a high-voltage field-effect transistor (FET) Q50. The FET Q50 hastwo main load terminals, which are coupled in series between the circuitcommon of the transmitting circuit 320 and the negative DC terminal ofthe bridge rectifier 340. The FET Q50 is preferably part number FQD6N50manufactured by Fairchild Semiconductor Corporation (which is rated fora maximum voltage of 500 V), but may comprise any suitable type ofcontrollably conductive device, such as relay or a semiconductor switch.The FET Q50 is conductive when the correct link voltage (i.e.,approximately 18 V_(DC)) is present across the communication link 110.The FET Q50 is operable to become non-conductive and disconnect thereceiving circuit 310 and the transmitting circuit 320 from thecommunication link 110 when the voltage across the conductors of thecommunication link exceeds a predetermined miswire threshold voltageV_(MISWIRE), e.g., 30 V.

The fault protection circuit 330 comprises a turn-on circuit including aresistor R52, a diode D54, a capacitor C56, and a zener diode Z58. Whenthe voltage across the communication link 110 is at the correct linkvoltage, a current flows through the resistor R52 and the diode D54 tocharge the capacitor C56 and generate a drive voltage V_(DRIVE). Thedrive voltage V_(DRIVE) developed across the capacitor C56 is limited bythe zener diode Z58, which preferably has a break-over voltage of 10 V.The drive voltage V_(DRIVE) across the capacitor C56 is provided to thegate of the FET Q50, causing the FET Q50 to be rendered conductive whenthe communication link 110 is at correct link voltage (i.e.,approximately 18 V_(DC)). The capacitor C56 maintains the FET Q50conductive while the communication link 110 is being shorted.Preferably, the resistor R52 has a resistance of 300 kΩ and thecapacitor C56 has a capacitance of 220 nF.

The fault protection circuit 330 also comprises a turn-off circuitincluding an NPN bipolar junction transistor Q60, a zener diode Z62, andtwo resistors R64, R66 (which preferably have resistances of 450 kΩ and100 kΩ, respectively). The transistor Q60 is coupled across thecapacitor C56 and is non-conductive when the link voltage is at thecorrect value of 18 V_(DC). However, when the voltage across thecommunication link 110 exceeds a predetermined miswire threshold, i.e.,the break-over voltage of the zener diode Z62 (e.g., preferably 24 V),the zener diode Z62 begins to conduct a current through the resistorsR64, R66. When a voltage produced across the resistor R66 exceeds theappropriate base-emitter voltage of the transistor Q60, the transistorQ60 begins to conduct, thus shorting the capacitor C56 and causing theFET Q50 to be rendered non-conductive.

The turn-off circuit is operable to render the transistor Q60 conductivebefore the turn-on circuit renders the FET Q50 conductive therebyprotecting the receiving circuit 310 and the transmitting circuit 320 inthe event of a high voltage miswire. The resistor R52 and the capacitorC56 of the turn-on circuit determine a time delay from when a voltage isapplied to the terminals E1, E2 of the communication circuit 280 to whenthe FET Q50 is rendered conductive. When a voltage above thepredetermined threshold is applied to the terminals E1, E2, the zenerdiode Z62 of the turn-off circuit causes the transistor Q60 to becomeconductive before the capacitor C56 charges to the appropriate voltageto turn on the FET Q50.

If the transmitting circuit 320 is shorting the communication link 10,i.e., the transistor Q28 is conductive when a high voltage miswire isapplied to the terminals E1, E2 of the communication circuit 280, thecurrent limit circuit of the transmitting circuit 320 protects thetransistor Q28. Specifically, in the event of a high voltage miswirewhile the transistor Q28 is conducting, the transistor Q42 limits thecurrent through the resistor R44 to approximately 300 milliamps.Accordingly, the voltage across the collector-emitter subsequentlyincreases while the fault protection circuit 330 disconnects thetransmitting circuit 320 from the terminal E2.

While the ballast 120 of the present invention is preferably a DALIballast for use with a DALI communication link, the communicationcircuit of the present invention could be used with control devicescoupled to other types of communication links using communicationprotocols other than the DALI protocol. Further, the fault protectioncircuit 330 could be used to protect any type of control circuit orcommunication circuit from high-voltage miswires.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

1. A communication circuit for a load control device, the load controldevice operable to be coupled to a communication link having twoconductors, the load control device operable to transmit a digitalmessage by alternating the communication link between an idle state inwhich a link voltage is developed across the conductors on thecommunication link and an active state in which the conductors of thecommunication link are substantially the same electrical potential, thecommunication circuit comprising: a receiving circuit coupled betweenthe conductors of the communication link, the receiving circuit operableto conduct an idle current when the communication link is in the idlestate; and a transmitting circuit coupled between the conductors of thecommunication link, the transmitting circuit comprising: an optocouplerhaving an input and an output, the output comprising a phototransistorand operable to become conductive when the input is driven with an inputcurrent, the output operable to conduct the idle current immediatelyafter becoming conductive; a voltage clamp coupled across the output ofthe optocoupler, the voltage clamp operable to clamp the voltage acrossthe output of the optocoupler in the idle state, wherein the voltageclamp limits the voltage across the output of the optocoupler toapproximately two diode drops; a controllably conductive deviceresponsive to the output of the optocoupler to electrically couple theconductors of the communication link together when the output of theoptocoupler is conductive and to stop electrically connecting theconductors of the communication link when the output is non-conductive;and a current source coupled to the phototransistor of the optocoupler,the current source operable to generate a source current when thecontrollably conductive device is electrically coupling the conductorsof the communication link together; wherein the output of theoptocoupler is operable to conduct a first portion of the source currentand the voltage clamp is operable to conduct a second portion of thesource current in the active state, such that the phototransistor of theoptocoupler is maintained in an active region of operation.
 2. Thecommunication circuit of claim 1, wherein the transmitting circuitfurther comprises an NPN bipolar junction transistor having a basecoupled to the emitter of the phototransistor, the series-combination ofthe collector-emitter junction of the phototransistor and thebase-emitter junction of the NPN transistor coupled in parallelelectrical connection with the voltage clamp, the NPN transistoroperable to become conductive when the phototransistor of theoptocoupler is conductive, the controllably conductive device responsiveto the NPN transistor to become conductive when the NPN transistor isconductive.
 3. The communication circuit of claim 2, wherein thecontrollably conductive device comprises a first PNP bipolar junctiontransistor and the transmitting circuit further comprises a diode havinga cathode coupled to the collector of the NPN transistor and an anodecoupled to a base of the first PNP transistor, such that when the NPNtransistor is conductive, a current is conducted through the base of thefirst PNP transistor.
 4. The communication circuit of claim 3, whereinthe series combination of the output of the optocoupler and the currentsource are coupled in series electrical connection between theconductors of the communication link.
 5. The communication circuit ofclaim 4, wherein the current source comprises a second PNP bipolartransistor having a collector coupled to the collector of thephototransistor of the optocoupler, a first resistor coupled to theemitter of the second PNP transistor, and a second resistor coupled inseries between the base of the second PNP transistor and the cathode ofthe diode, the series combination of the first resistor, theemitter-base junction of the second PNP transistor, and the secondresistor coupled in parallel electrical connection with the seriescombination of the emitter-base junction of the first PNP transistor andthe diode.
 6. The communication circuit of claim 5, wherein the sourcecurrent is generated by the first resistor and provided to thephototransistor of the optocoupler through the second PNP transistorwhen the first PNP transistor is conductive.
 7. The communicationcircuit of claim 6, wherein the source current is approximately fivemilliamps, and the first portion of the source current conducted throughthe phototransistor of the optocoupler is approximately two milliamps.8. The communication circuit of claim 2, wherein when thephototransistor is conductive, the voltage across the output of theoptocoupler is equal to approximately one diode drop.
 9. Thecommunication circuit of claim 8, wherein the voltage across the outputof the optocoupler is equal to approximately two diode drops, such thatthe voltage swing across the output of the optocoupler is equal toapproximately one diode drop.
 10. The communication circuit of claim 1,wherein the voltage clamp comprises two series-connected diodes.
 11. Thecommunication circuit of claim 1, wherein the controllably conductivedevice comprises a semiconductor switch.
 12. The communication circuitof claim 11, wherein the semiconductor switch comprises a bipolarjunction transistor.
 13. The communication circuit of claim 1, whereinthe voltage clamp comprises a zener diode.
 14. The communication circuitof claim 1, wherein a rise time of the digital message is approximately10-15 μsec.
 15. A high-voltage fault protection circuit for protecting acommunication circuit of a load control device coupled to a digitalcommunication link via first and second terminals, the fault protectioncircuit operatively coupled to the first and second terminals of theload control device, the fault protection circuit comprising: acontrollably conductive device having two main load terminals and acontrol input, the main load terminals coupled in series electricalconnection between the communication circuit and the second terminal ofthe load control device, the controllably conductive device renderedconductive when a first voltage having a magnitude less than apredetermined threshold is provided across the first and secondterminals, the controllably conductive device operable to becomenon-conductive and to disconnect the communication circuit and thesecond terminal when a second voltage having a magnitude greater thanthe predetermined threshold is provided across the first and secondterminals; and a turn-on circuit coupled between the first terminal andthe control input of the controllably conductive device, the turn-oncircuit operable to provide a drive voltage to the control input of thecontrollably conductive device when the first voltage is provided acrossthe first and second terminals, wherein the turn-on circuit comprises aresistor, a capacitor, and a zener diode, the capacitor coupled to thecontrol input of the controllably conductive device such that the drivevoltage develops across the capacitor, the zener diode coupled inparallel electrical connection with the capacitor to limit the magnitudeof the drive voltage.
 16. The fault protection circuit of claim 15,further comprising: a turn-off circuit coupled between the firstterminal and the control input of the controllably conductive device,the turn-off circuit operable to prevent the delivery of the drivevoltage to the control input of the controllably conductive device whenthe second voltage is provided across the first and second terminals.17. The fault protection circuit of claim 16, wherein the turn-offcircuit comprises a transistor coupled to the control input of thecontrollably conductive device, and a zener diode coupled between thefirst terminal and a base of the transistor, such that the zener diodeis operable to conduct a current into the base of the transistor whenthe second voltage is provided across the first and second terminals,whereby the transistor becomes conductive to remove the drive voltagefrom the control input of the controllably conductive device.
 18. Thefault protection circuit of claim 15, wherein the controllablyconductive device comprises a semiconductor switch.
 19. The faultprotection circuit of claim 18, wherein the semiconductor switchcomprises a high-voltage field-effect transistor.
 20. A communicationcircuit for a load control device, the load control device operable tobe coupled to a communication link having two conductors, the loadcontrol device operable to transmit a digital message by changing thecommunication link between an idle state in which a first voltage isdeveloped across the conductors on the communication link and an activestate in which the conductors of the communication link are atsubstantially the same electrical potential, the communication circuitcomprising: a receiving circuit coupled between the conductors of thecommunication link, the receiving circuit operable to conduct an idlecurrent; a transmitting circuit coupled between the conductors of thecommunication link, the transmitting circuit comprising an optocouplerhaving a phototransistor for providing an output, a voltage clampoperable to clamp the voltage across the output of the optocoupler inthe idle state, wherein the voltage clamp limits the voltage across theoutput of the optocoupler to approximately two diode drops, acontrollably conductive device responsive to the output of theoptocoupler to electrically couple the conductors of the communicationlink together when the output of the optocoupler is conductive, and acurrent source operable to provide the phototransistor of theoptocoupler with excess current such that the phototransistor ismaintained in the active region when the phototransistor is conductive;and a fault protection circuit operatively coupled between theconductors of the communication link and operable to protect thereceiving and transmitting circuits, the fault protection circuitcomprising a controllably conductive device coupled between thereceiving and transmitting circuits and a second one of conductors ofthe communication link, the controllably conductive device renderedconductive when the first voltage is provided across the conductors ofthe communication link, the controllably conductive device renderednon-conductive when a second voltage is provided across the first andsecond terminals, the first voltage having a magnitude less than apredetermined threshold, the second voltage having a magnitude greaterthan the predetermined threshold.
 21. The communication circuit of claim20, wherein the transmitting circuit further comprises a current limitcircuit for limiting the magnitude of the current through thecontrollably conductive device.
 22. The communication circuit of claim21, wherein if the controllably conductive device is conductive and thesecond voltage is provided across the first and second terminals, thecurrent limit circuit limits the magnitude of the current through thecontrollably conductive device, the voltage across the controllablyconductive device increases, and the fault protection circuitsubsequently disconnects the transmitting circuit from the second one ofthe conductors of the communication link.
 23. A method of transmitting adigital message from a communication circuit via a communication linkhaving two conductors, the method comprising the steps of: drawing anidle current when the communication link is in an idle state; providingan optocoupler having an input and an output comprising aphototransistor; limiting the voltage produced across the output of theoptocoupler to approximately two diode drops; driving the input of theoptocoupler, such that the output of the optocoupler is operable toconduct a drive current; electrically connecting the two conductors ofthe communication link to change the communication link from an idlestate to a shorted state in response to the step of driving the input ofthe optocoupler; providing a source current to the phototransistor tomaintain the phototransistor in the active region of operation when thephototransistor is conducting the drive current; and ceasing driving theinput of the optocoupler, such that the output of the optocoupler ceasesto conduct the drive current.
 24. The method of claim 23, wherein thedrive current has an initial magnitude substantially the same as themagnitude of the idle current.
 25. The method of claim 23, wherein themagnitude of the source current is greater than the magnitude of thedrive current, the method further comprising the step of: providing acurrent path for the excess current equal to the difference between thesource current and the drive current.